One time, when I came out to the Bay Area to visit Natalie, we decided to have a date at a restaurant on the edge of Golden Gate park. I unwisely suggested that we could take the train up to the city and then a cab out to the restaurant.
The plan went well as far as getting us to the place but when it came to getting back, we ran into a snag. We were so far off of the profitable routes that we couldn't find a cab to pick us up and drive us back to the train station (in spite of the fact that various taxi companies assured us that cabbies were on the way -- assertions that required us to stand out in a miserably cold drizzle for well over an hour).
Eventually a very nice young couple (whom I would happily include in my will if I had saved their names) took pity on us and drove us back to the station.
Now that I've burdened you with that little anecdote, contemplate this Quicktime visualization of taxi traffic patterns in San Francisco over a 4 hour period.
Please be warned that the file is large and can take some time to load if you don't have good bandwidth.